// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/check/eval_inst.h"

#include <variant>

#include "toolchain/base/kind_switch.h"
#include "toolchain/check/action.h"
#include "toolchain/check/cpp/constant.h"
#include "toolchain/check/cpp/import.h"
#include "toolchain/check/cpp/type_mapping.h"
#include "toolchain/check/diagnostic_helpers.h"
#include "toolchain/check/facet_type.h"
#include "toolchain/check/generic.h"
#include "toolchain/check/impl_lookup.h"
#include "toolchain/check/import_ref.h"
#include "toolchain/check/inst.h"
#include "toolchain/check/period_self.h"
#include "toolchain/check/type.h"
#include "toolchain/check/type_completion.h"
#include "toolchain/diagnostics/diagnostic.h"
#include "toolchain/parse/typed_nodes.h"
#include "toolchain/sem_ir/builtin_function_kind.h"
#include "toolchain/sem_ir/expr_info.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/pattern.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

// Performs an access into an aggregate, retrieving the specified element.
static auto PerformAggregateAccess(Context& context, SemIR::Inst inst)
    -> ConstantEvalResult {
  auto access_inst = inst.As<SemIR::AnyAggregateAccess>();
  if (auto aggregate = context.insts().TryGetAs<SemIR::AnyAggregateValue>(
          access_inst.aggregate_id)) {
    auto elements = context.inst_blocks().Get(aggregate->elements_id);
    auto index = static_cast<size_t>(access_inst.index.index);
    CARBON_CHECK(index < elements.size(), "Access out of bounds.");
    // `Phase` is not used here. If this element is a concrete constant, then
    // so is the result of indexing, even if the aggregate also contains a
    // symbolic context.
    return ConstantEvalResult::Existing(
        context.constant_values().Get(elements[index]));
  }

  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& /*context*/, SemIR::ArrayInit inst)
    -> ConstantEvalResult {
  // TODO: Add an `ArrayValue` to represent a constant array object
  // representation instead of using a `TupleValue`.
  return ConstantEvalResult::NewSamePhase(
      SemIR::TupleValue{.type_id = inst.type_id, .elements_id = inst.inits_id});
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::ArrayType inst) -> ConstantEvalResult {
  auto bound_inst = context.insts().Get(inst.bound_id);
  auto int_bound = bound_inst.TryAs<SemIR::IntValue>();
  if (!int_bound) {
    CARBON_CHECK(context.constant_values().Get(inst.bound_id).is_symbolic(),
                 "Unexpected inst {0} for template constant int", bound_inst);
    return ConstantEvalResult::NewSamePhase(inst);
  }
  // TODO: We should check that the size of the resulting array type
  // fits in 64 bits, not just that the bound does. Should we use a
  // 32-bit limit for 32-bit targets?
  const auto& bound_val = context.ints().Get(int_bound->int_id);
  if (context.types().IsSignedInt(int_bound->type_id) &&
      bound_val.isNegative()) {
    CARBON_DIAGNOSTIC(ArrayBoundNegative, Error,
                      "array bound of {0} is negative", TypedInt);
    context.emitter().Emit(
        context.insts().GetAs<SemIR::ArrayType>(inst_id).bound_id,
        ArrayBoundNegative, {.type = int_bound->type_id, .value = bound_val});
    return ConstantEvalResult::Error;
  }
  if (bound_val.getActiveBits() > 64) {
    CARBON_DIAGNOSTIC(ArrayBoundTooLarge, Error,
                      "array bound of {0} is too large", TypedInt);
    context.emitter().Emit(
        context.insts().GetAs<SemIR::ArrayType>(inst_id).bound_id,
        ArrayBoundTooLarge, {.type = int_bound->type_id, .value = bound_val});
    return ConstantEvalResult::Error;
  }
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::AsCompatible inst)
    -> ConstantEvalResult {
  // AsCompatible changes the type of the source instruction; its constant
  // value, if there is one, needs to be modified to be of the same type.
  auto value_id = context.constant_values().Get(inst.source_id);
  CARBON_CHECK(value_id.is_constant());

  auto value_inst =
      context.insts().Get(context.constant_values().GetInstId(value_id));
  value_inst.SetType(inst.type_id);
  return ConstantEvalResult::NewAnyPhase(value_inst);
}

auto EvalConstantInst(Context& context, SemIR::AliasBinding inst)
    -> ConstantEvalResult {
  // An alias evaluates to the value it's bound to.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.value_id));
}

auto EvalConstantInst(Context& context, SemIR::RefBinding inst)
    -> ConstantEvalResult {
  // A reference binding evaluates to the value it's bound to.
  if (inst.value_id.has_value()) {
    return ConstantEvalResult::Existing(
        context.constant_values().Get(inst.value_id));
  }
  return ConstantEvalResult::NotConstant;
}

auto EvalConstantInst(Context& /*context*/, SemIR::ValueBinding /*inst*/)
    -> ConstantEvalResult {
  // Non-`:!` value bindings are not constant.
  return ConstantEvalResult::NotConstant;
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::AcquireValue inst) -> ConstantEvalResult {
  SemIR::ConstantId const_id = SemIR::ConstantId::NotConstant;
  if (const auto* var_decl = GetAsClangVarDecl(context, inst.value_id)) {
    const_id =
        EvalCppVarDecl(context, SemIR::LocId(inst_id), var_decl, inst.type_id);
  } else if (auto temporary =
                 context.insts().TryGetAs<SemIR::Temporary>(inst.value_id)) {
    const_id = context.constant_values().Get(temporary->init_id);
  }

  if (const_id.has_value() && const_id.is_constant()) {
    return ConstantEvalResult::Existing(const_id);
  }

  return ConstantEvalResult::NotConstant;
}

auto EvalConstantInst(Context& context, SemIR::ClassElementAccess inst)
    -> ConstantEvalResult {
  return PerformAggregateAccess(context, inst);
}

auto EvalConstantInst(Context& context, SemIR::ClassDecl inst)
    -> ConstantEvalResult {
  const auto& class_info = context.classes().Get(inst.class_id);

  // If the class has generic parameters, we don't produce a class type, but a
  // callable whose return value is a class type.
  if (class_info.has_parameters()) {
    return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
        .type_id = inst.type_id, .elements_id = SemIR::InstBlockId::Empty});
  }

  // A non-generic class declaration evaluates to the class type.
  return ConstantEvalResult::NewAnyPhase(SemIR::ClassType{
      .type_id = SemIR::TypeType::TypeId,
      .class_id = inst.class_id,
      .specific_id =
          context.generics().GetSelfSpecific(class_info.generic_id)});
}

auto EvalConstantInst(Context& /*context*/, SemIR::ClassInit inst)
    -> ConstantEvalResult {
  // TODO: Add a `ClassValue` to represent a constant class object
  // representation instead of using a `StructValue`.
  return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
      .type_id = inst.type_id, .elements_id = inst.elements_id});
}

auto EvalConstantInst(Context& context, SemIR::ConstType inst)
    -> ConstantEvalResult {
  // `const (const T)` evaluates to `const T`.
  if (context.insts().Is<SemIR::ConstType>(inst.inner_id)) {
    return ConstantEvalResult::Existing(
        context.constant_values().Get(inst.inner_id));
  }
  // Otherwise, `const T` evaluates to itself.
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& /*context*/, SemIR::PartialType inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::Converted inst)
    -> ConstantEvalResult {
  // A conversion evaluates to the result of the conversion.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.result_id));
}

auto EvalConstantInst(Context& /*context*/, SemIR::Deref /*inst*/)
    -> ConstantEvalResult {
  // TODO: Handle this.
  return ConstantEvalResult::TODO;
}

auto EvalConstantInst(Context& context, SemIR::ExportDecl inst)
    -> ConstantEvalResult {
  // An export instruction evaluates to the exported declaration.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.value_id));
}

auto EvalConstantInst(Context& context, SemIR::FacetAccessType inst)
    -> ConstantEvalResult {
  if (auto facet_value = context.insts().TryGetAs<SemIR::FacetValue>(
          inst.facet_value_inst_id)) {
    return ConstantEvalResult::Existing(
        context.constant_values().Get(facet_value->type_inst_id));
  }

  // The `facet_value_inst_id` is always a facet value (has type facet type).
  CARBON_CHECK(context.types().Is<SemIR::FacetType>(
      context.insts().Get(inst.facet_value_inst_id).type_id()));

  // Other instructions (e.g. ImplWitnessAccess) of type FacetType can appear
  // here, in which case the constant inst is a FacetAccessType until those
  // instructions resolve to one of the above.
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::FacetValue inst)
    -> ConstantEvalResult {
  // A FacetValue that just wraps a facet without adding/removing any witnesses
  // (which means they have the same type) is evaluated to the facet itself.
  if (auto access =
          context.insts().TryGetAs<SemIR::FacetAccessType>(inst.type_inst_id)) {
    if (context.insts().Get(access->facet_value_inst_id).type_id() ==
        inst.type_id) {
      return ConstantEvalResult::Existing(
          context.constant_values().Get(access->facet_value_inst_id));
    }
  }

  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::FloatType inst) -> ConstantEvalResult {
  return ValidateFloatTypeAndSetKind(context, SemIR::LocId(inst_id), inst)
             ? ConstantEvalResult::NewSamePhase(inst)
             : ConstantEvalResult::Error;
}

auto EvalConstantInst(Context& /*context*/, SemIR::FunctionDecl inst)
    -> ConstantEvalResult {
  // A function declaration evaluates to a function object, which is an empty
  // object of function type.
  // TODO: Eventually we may need to handle captures here.
  return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
      .type_id = inst.type_id, .elements_id = SemIR::InstBlockId::Empty});
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::LookupImplWitness inst) -> ConstantEvalResult {
  // Canonicalize the query self to reduce the number of unique witness
  // instructions and enable constant value comparisons.
  auto self_facet_value_inst_id = SemIR::InstId::None;
  inst.query_self_inst_id = context.constant_values().GetInstId(
      GetCanonicalQuerySelfForLookupImplWitness(
          context, context.constant_values().Get(inst.query_self_inst_id),
          &self_facet_value_inst_id));

  auto witness_id = EvalLookupSingleFinalWitness(context, SemIR::LocId(inst_id),
                                                 inst, self_facet_value_inst_id,
                                                 EvalImplLookupMode::Normal);
  if (witness_id == SemIR::ErrorInst::ConstantId) {
    return ConstantEvalResult::Error;
  }
  if (witness_id.has_value()) {
    return ConstantEvalResult::Existing(witness_id);
  }
  // Try again when the query is modified by a specific.
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::ImplWitnessAccess inst) -> ConstantEvalResult {
  CARBON_DIAGNOSTIC(ImplAccessMemberBeforeSet, Error,
                    "accessing member from impl before it has a defined value");
  CARBON_KIND_SWITCH(context.insts().Get(inst.witness_id)) {
    case CARBON_KIND(SemIR::ImplWitness witness): {
      // This is PerformAggregateAccess followed by GetConstantValueInSpecific.
      auto witness_table = context.insts().GetAs<SemIR::ImplWitnessTable>(
          witness.witness_table_id);
      auto elements = context.inst_blocks().Get(witness_table.elements_id);
      // `elements` can be empty if there is only a forward declaration of the
      // impl.
      if (!elements.empty()) {
        auto index = static_cast<size_t>(inst.index.index);
        CARBON_CHECK(index < elements.size(), "Access out of bounds.");
        auto element = elements[index];
        if (element.has_value()) {
          LoadImportRef(context, element);
          return ConstantEvalResult::Existing(GetConstantValueInSpecific(
              context.sem_ir(), witness.specific_id, element));
        }
      }
      // If we get here, this impl witness table entry has not been populated
      // yet, because the impl was referenced within its own definition.
      // TODO: Add note pointing to the impl declaration.
      context.emitter().Emit(inst_id, ImplAccessMemberBeforeSet);
      return ConstantEvalResult::Error;
    }
    case CARBON_KIND(SemIR::CustomWitness custom_witness): {
      auto elements = context.inst_blocks().Get(custom_witness.elements_id);
      auto index = static_cast<size_t>(inst.index.index);
      // `elements` can be shorter than the number of associated entities while
      // we're building the synthetic witness.
      if (index < elements.size()) {
        return ConstantEvalResult::Existing(
            context.constant_values().Get(elements[index]));
      }
      // If we get here, this synthesized witness table entry has not been
      // populated yet.
      // TODO: Is this reachable? We have no test coverage for this diagnostic.
      context.emitter().Emit(inst_id, ImplAccessMemberBeforeSet);
      return ConstantEvalResult::Error;
    }
    case CARBON_KIND(SemIR::LookupImplWitness witness): {
      // If the witness is symbolic but has a self type that is a FacetType, it
      // can pull rewrite values from the self's facet type. If the access is
      // for one of those rewrites, evaluate to the RHS of the rewrite.

      // The type of the query self type (a FacetType or TypeType).
      auto access_self_type_id =
          context.insts().Get(witness.query_self_inst_id).type_id();
      if (context.types().Is<SemIR::TypeType>(access_self_type_id)) {
        // A self facet of type `type` has no rewrite constraints to look in.
        return ConstantEvalResult::NewSamePhase(inst);
      }

      // The `ImplWitnessAccess` is accessing a value, by index, for this
      // `self impls interface` combination.
      auto access_self =
          context.constant_values().Get(witness.query_self_inst_id);
      auto access_interface = context.specific_interfaces().Get(
          witness.query_specific_interface_id);

      auto access_self_facet_type_id =
          context.types()
              .GetAs<SemIR::FacetType>(access_self_type_id)
              .facet_type_id;
      // TODO: We could consider something better than linear search here, such
      // as a map. However that would probably require heap allocations which
      // may be worse overall since the number of rewrite constraints is
      // generally low. If the `rewrite_constraints` were sorted so that
      // associated constants are grouped together, as in
      // ResolveFacetTypeRewriteConstraints(), and limited to just the
      // `ImplWitnessAccess` entries, then a binary search may work here.
      for (const auto& rewrite : context.facet_types()
                                     .Get(access_self_facet_type_id)
                                     .rewrite_constraints) {
        // Look at each rewrite constraint in the self facet's type. If the LHS
        // is an `ImplWitnessAccess` into the same interface that `inst` is
        // indexing into, then we can use its RHS as the value.
        auto rewrite_lhs_access =
            context.insts().TryGetAs<SemIR::ImplWitnessAccess>(rewrite.lhs_id);
        if (!rewrite_lhs_access) {
          continue;
        }
        if (rewrite_lhs_access->index != inst.index) {
          continue;
        }

        // Witnesses come from impl lookup, and the operands are from
        // IdentifiedFacetTypes, so `.Self` is replaced. However rewrite
        // constraints are not part of an IdentifiedFacetType, so they are not
        // replaced. We have to do the same replacement in the rewrite's LHS
        // witness in order to compare it with the access witness.
        //
        // However we don't substitute the witness directly as that would
        // re-evaluate it and cause us to do an impl lookup. Instead we
        // substitute and compare its operands.
        auto rewrite_lhs_witness =
            context.insts().GetAs<SemIR::LookupImplWitness>(
                rewrite_lhs_access->witness_id);

        SubstPeriodSelfCallbacks callbacks(&context, SemIR::LocId(inst_id),
                                           access_self);

        auto rewrite_lhs_self = context.constant_values().Get(
            rewrite_lhs_witness.query_self_inst_id);
        rewrite_lhs_self =
            SubstPeriodSelf(context, callbacks, rewrite_lhs_self);
        // Witnesses have a canonicalized self value. Perform the same
        // canonicalization here so that we can compare them.
        rewrite_lhs_self = GetCanonicalQuerySelfForLookupImplWitness(
            context, rewrite_lhs_self);

        if (rewrite_lhs_self != access_self) {
          // This rewrite is into a different self type than the access query.
          continue;
        }

        auto rewrite_lhs_interface = SubstPeriodSelf(
            context, callbacks,
            context.specific_interfaces().Get(
                rewrite_lhs_witness.query_specific_interface_id));

        if (rewrite_lhs_interface != access_interface) {
          // This rewrite is into a different interface than the access query.
          continue;
        }

        // The `ImplWitnessAccess` evaluates to the RHS from the witness self
        // facet value's type. Any `.Self` references in the RHS are also
        // replaced with the self type of the access.
        auto rewrite_rhs = SubstPeriodSelf(
            context, callbacks, context.constant_values().Get(rewrite.rhs_id));
        return ConstantEvalResult::Existing(rewrite_rhs);
      }
      break;
    }
    default:
      break;
  }
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context,
                      SemIR::ImplWitnessAccessSubstituted inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.value_id));
}

auto EvalConstantInst(Context& context,
                      SemIR::ImplWitnessAssociatedConstant inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.inst_id));
}

auto EvalConstantInst(Context& /*context*/, SemIR::ImportRefUnloaded inst)
    -> ConstantEvalResult {
  CARBON_FATAL("ImportRefUnloaded should be loaded before TryEvalInst: {0}",
               inst);
}

auto EvalConstantInst(Context& context, SemIR::InPlaceInit inst)
    -> ConstantEvalResult {
  // Initialization is not performed in-place during constant evaluation, so
  // just return the value of the initializer.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.src_id));
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::IntType inst) -> ConstantEvalResult {
  return ValidateIntType(context, SemIR::LocId(inst_id), inst)
             ? ConstantEvalResult::NewSamePhase(inst)
             : ConstantEvalResult::Error;
}

auto EvalConstantInst(Context& context, SemIR::InterfaceDecl inst)
    -> ConstantEvalResult {
  const auto& interface_info = context.interfaces().Get(inst.interface_id);

  // If the interface has generic parameters, we don't produce an interface
  // type, but a callable whose return value is an interface type.
  if (interface_info.has_parameters()) {
    return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
        .type_id = inst.type_id, .elements_id = SemIR::InstBlockId::Empty});
  }

  // A non-parameterized interface declaration evaluates to a declared facet
  // type containing just the interface.
  return ConstantEvalResult::NewAnyPhase(FacetTypeFromInterface(
      context, inst.interface_id,
      context.generics().GetSelfSpecific(interface_info.generic_id)));
}

auto EvalConstantInst(Context& context, SemIR::MarkInPlaceInit inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.src_id));
}

auto EvalConstantInst(Context& context, SemIR::NamedConstraintDecl inst)
    -> ConstantEvalResult {
  const auto& named_constraint_info =
      context.named_constraints().Get(inst.named_constraint_id);

  // If the named constraint has generic parameters, we don't produce a named
  // constraint type, but a callable whose return value is a named constraint
  // type.
  if (named_constraint_info.has_parameters()) {
    return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
        .type_id = inst.type_id, .elements_id = SemIR::InstBlockId::Empty});
  }

  // A non-parameterized named constraint declaration evaluates to a declared
  // facet type containing just the named constraint.
  return ConstantEvalResult::NewAnyPhase(FacetTypeFromNamedConstraint(
      context, inst.named_constraint_id,
      context.generics().GetSelfSpecific(named_constraint_info.generic_id)));
}

auto EvalConstantInst(Context& context, SemIR::NameRef inst)
    -> ConstantEvalResult {
  // A name reference evaluates to the value the name resolves to.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.value_id));
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::RequireCompleteType inst) -> ConstantEvalResult {
  auto witness_type_id =
      GetSingletonType(context, SemIR::WitnessType::TypeInstId);

  // If the type is a concrete constant, require it to be complete now.
  auto complete_type_id =
      context.types().GetTypeIdForTypeInstId(inst.complete_type_inst_id);
  if (complete_type_id.is_concrete()) {
    Diagnostics::ContextScope diagnostic_context(
        &context.emitter(), [&](auto& builder) {
          CARBON_DIAGNOSTIC(IncompleteTypeInMonomorphization, Context,
                            "{0} evaluates to incomplete type {1}",
                            InstIdAsType, InstIdAsType);
          builder.Context(inst_id, IncompleteTypeInMonomorphization,
                          context.insts()
                              .GetAs<SemIR::RequireCompleteType>(inst_id)
                              .complete_type_inst_id,
                          inst.complete_type_inst_id);
        });
    // We use TryToCompleteType() instead of RequireCompleteType() because we
    // are currently evaluating a RequireCompleteType instruction, and calling
    // RequireCompleteType() would insert another copy of the same instruction.
    if (!TryToCompleteType(context, complete_type_id, SemIR::LocId(inst_id),
                           true)) {
      return ConstantEvalResult::Error;
    }
    return ConstantEvalResult::NewAnyPhase(SemIR::CompleteTypeWitness{
        .type_id = witness_type_id,
        .object_repr_type_inst_id = context.types().GetTypeInstId(
            context.types().GetObjectRepr(complete_type_id))});
  }

  // If it's not a concrete constant, require it to be complete once it
  // becomes one.
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::RequireSpecificDefinition inst)
    -> ConstantEvalResult {
  // This can return false, we just need to try it.
  ResolveSpecificDefinition(context, SemIR::LocId::None, inst.specific_id);
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::SpecificConstant inst)
    -> ConstantEvalResult {
  // Pull the constant value out of the specific.
  return ConstantEvalResult::Existing(SemIR::GetConstantValueInSpecific(
      context.sem_ir(), inst.specific_id, inst.inst_id));
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::SpecificImplFunction inst) -> ConstantEvalResult {
  auto callee_inst = context.insts().Get(inst.callee_id);
  // If the callee is not a function value, we're not ready to evaluate this
  // yet. Build a symbolic `SpecificImplFunction` constant.
  if (!callee_inst.Is<SemIR::StructValue>()) {
    return ConstantEvalResult::NewSamePhase(inst);
  }
  auto callee_type_id = callee_inst.type_id();
  auto callee_fn_type =
      context.types().TryGetAs<SemIR::FunctionType>(callee_type_id);
  if (!callee_fn_type) {
    return ConstantEvalResult::NewSamePhase(inst);
  }

  // If the callee function found in the impl witness is not generic, the result
  // is simply that function.
  // TODO: We could do this even before the callee is concrete.
  auto generic_id =
      context.functions().Get(callee_fn_type->function_id).generic_id;
  if (!generic_id.has_value()) {
    return ConstantEvalResult::Existing(
        context.constant_values().Get(inst.callee_id));
  }

  // Find the arguments to use.
  auto enclosing_specific_id = callee_fn_type->specific_id;
  auto enclosing_args = context.inst_blocks().Get(
      context.specifics().GetArgsOrEmpty(enclosing_specific_id));
  auto interface_fn_args = context.inst_blocks().Get(
      context.specifics().GetArgsOrEmpty(inst.specific_id));

  // Form new specific for the generic callee function. The arguments for this
  // specific are the enclosing arguments of the callee followed by the
  // remaining arguments from the interface function. Impl checking has ensured
  // that these arguments can also be used for the function in the impl witness.
  auto num_params = context.inst_blocks()
                        .Get(context.generics().Get(generic_id).bindings_id)
                        .size();
  llvm::SmallVector<SemIR::InstId> args;
  args.reserve(num_params);
  args.append(enclosing_args.begin(), enclosing_args.end());
  int remaining_params = num_params - args.size();
  CARBON_CHECK(static_cast<int>(interface_fn_args.size()) >= remaining_params);
  args.append(interface_fn_args.end() - remaining_params,
              interface_fn_args.end());
  auto specific_id =
      MakeSpecific(context, SemIR::LocId(inst_id), generic_id, args);
  context.definitions_required_by_use().push_back(
      {SemIR::LocId(inst_id), specific_id});

  return ConstantEvalResult::NewSamePhase(
      SemIR::SpecificFunction{.type_id = inst.type_id,
                              .callee_id = inst.callee_id,
                              .specific_id = specific_id});
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::SpecificFunction inst) -> ConstantEvalResult {
  auto callee_function =
      SemIR::GetCalleeAsFunction(context.sem_ir(), inst.callee_id);
  const auto& fn = context.functions().Get(callee_function.function_id);
  if (!callee_function.self_type_id.has_value() &&
      fn.builtin_function_kind() != SemIR::BuiltinFunctionKind::NoOp &&
      fn.virtual_modifier != SemIR::Function::VirtualModifier::Abstract) {
    // This is not an associated function. Those will be required to be defined
    // as part of checking that the impl is complete.
    context.definitions_required_by_use().push_back(
        {SemIR::LocId(inst_id), inst.specific_id});
  }
  // Create new constant for a specific function.
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::SpliceBlock inst)
    -> ConstantEvalResult {
  // SpliceBlock evaluates to the result value that is (typically) within the
  // block. This can be constant even if the block contains other non-constant
  // instructions.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.result_id));
}

auto EvalConstantInst(Context& context, SemIR::SpliceInst inst)
    -> ConstantEvalResult {
  // The constant value of a SpliceInst is the constant value of the instruction
  // being spliced. Note that `inst.inst_id` is the instruction being spliced,
  // so we need to go through another round of obtaining the constant value in
  // addition to the one performed by the eval infrastructure.
  auto nested_inst = context.insts().Get(inst.inst_id);
  if (auto inst_value = nested_inst.TryAs<SemIR::InstValue>()) {
    return ConstantEvalResult::Existing(
        context.constant_values().Get(inst_value->inst_id));
  }
  switch (nested_inst.kind().constant_kind()) {
    case SemIR::InstConstantKind::ConstantInstAction:
      return ConstantEvalResult::NewSamePhase(inst);
    case SemIR::InstConstantKind::InstAction:
      // TODO: Consider creating a new `ValueOfInst` instruction analogous to
      // `TypeOfInst` to defer determining the constant value until we know the
      // instruction. Alternatively, produce a symbolic `SpliceInst` constant.
      return ConstantEvalResult::NotConstant;
    default:
      CARBON_FATAL("Unexpected inst kind for inst splice: {0}", nested_inst);
  }
}

auto EvalConstantInst(Context& context, SemIR::StructAccess inst)
    -> ConstantEvalResult {
  return PerformAggregateAccess(context, inst);
}

auto EvalConstantInst(Context& /*context*/, SemIR::StructInit inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
      .type_id = inst.type_id, .elements_id = inst.elements_id});
}

auto EvalConstantInst(Context& /*context*/, SemIR::StructLiteral inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::NewSamePhase(SemIR::StructValue{
      .type_id = inst.type_id, .elements_id = inst.elements_id});
}

auto EvalConstantInst(Context& context, SemIR::TupleAccess inst)
    -> ConstantEvalResult {
  return PerformAggregateAccess(context, inst);
}

auto EvalConstantInst(Context& /*context*/, SemIR::TupleInit inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::NewSamePhase(SemIR::TupleValue{
      .type_id = inst.type_id, .elements_id = inst.elements_id});
}

auto EvalConstantInst(Context& /*context*/, SemIR::TupleLiteral inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::NewSamePhase(SemIR::TupleValue{
      .type_id = inst.type_id, .elements_id = inst.elements_id});
}

auto EvalConstantInst(Context& context, SemIR::TypeComponentOf inst)
    -> ConstantEvalResult {
  auto form_constant_inst_id =
      context.constant_values().GetConstantInstId(inst.form_inst_id);
  if (auto primitive_form = context.insts().TryGetAs<SemIR::AnyPrimitiveForm>(
          form_constant_inst_id)) {
    return ConstantEvalResult::Existing(
        context.constant_values().Get(primitive_form->type_component_id));
  }
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::TypeLiteral inst)
    -> ConstantEvalResult {
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.value_id));
}

auto EvalConstantInst(Context& context, SemIR::TypeOfInst inst)
    -> ConstantEvalResult {
  // Grab the type from the instruction produced as our operand.
  if (auto inst_value =
          context.insts().TryGetAs<SemIR::InstValue>(inst.inst_id)) {
    return ConstantEvalResult::Existing(context.types().GetConstantId(
        context.insts().Get(inst_value->inst_id).type_id()));
  }
  return ConstantEvalResult::NewSamePhase(inst);
}

auto EvalConstantInst(Context& context, SemIR::UnaryOperatorNot inst)
    -> ConstantEvalResult {
  // `not true` -> `false`, `not false` -> `true`.
  // All other uses of unary `not` are non-constant.
  auto const_id = context.constant_values().Get(inst.operand_id);
  if (const_id.is_concrete()) {
    auto value =
        context.constant_values().GetInstAs<SemIR::BoolLiteral>(const_id);
    value.value = SemIR::BoolValue::From(!value.value.ToBool());
    return ConstantEvalResult::NewSamePhase(value);
  }
  return ConstantEvalResult::NotConstant;
}

auto EvalConstantInst(Context& /*context*/, SemIR::UpdateInit /*inst*/)
    -> ConstantEvalResult {
  // TODO: Support folding together a ClassInit with an update that sets the
  // vptr.
  return ConstantEvalResult::TODO;
}

auto EvalConstantInst(Context& context, SemIR::ValueOfInitializer inst)
    -> ConstantEvalResult {
  // Values of value expressions and initializing expressions are represented in
  // the same way during constant evaluation, so just return the value of the
  // operand.
  return ConstantEvalResult::Existing(
      context.constant_values().Get(inst.init_id));
}

auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                      SemIR::VarStorage inst) -> ConstantEvalResult {
  if (!inst.pattern_id.has_value()) {
    // This variable was not created from a `var` pattern, so isn't a global
    // variable.
    return ConstantEvalResult::NotConstant;
  }

  // A variable is constant if it's global.
  auto entity_name_id = SemIR::GetFirstBindingNameFromPatternId(
      context.sem_ir(), inst.pattern_id);
  if (!entity_name_id.has_value()) {
    // Variable doesn't introduce any bindings, so can only be referenced by its
    // own initializer. We treat such a reference as not being constant.
    return ConstantEvalResult::NotConstant;
  }

  auto scope_id = context.entity_names().Get(entity_name_id).parent_scope_id;
  if (!scope_id.has_value()) {
    return ConstantEvalResult::NotConstant;
  }
  auto scope_inst =
      context.insts().Get(context.name_scopes().Get(scope_id).inst_id());
  if (!scope_inst.Is<SemIR::Namespace>() &&
      !scope_inst.Is<SemIR::ClassDecl>()) {
    // Only namespace-scope and class-scope variables are reference constants.
    // Class-scope variables cannot currently be declared directly, but can
    // occur when static data members are imported from C++.
    return ConstantEvalResult::NotConstant;
  }

  // This is a constant reference expression denoting this global variable.
  return ConstantEvalResult::Existing(
      SemIR::ConstantId::ForConcreteConstant(inst_id));
}

}  // namespace Carbon::Check